clk: rockchip: use the dclk_vop_frac clock ids on rk3399

clk: rockchip: use the dclk_vop_frac clock ids on rk3399

Export the dclk_vop_frac out, so we can set the dclk_vop as the
child of dclk_vop_frac, and then we can start to take use of
the fractional dividers.

Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>